Recently, there has been a significant increase in the demand for renewable energy sources with solar energy being viewed as one of the most reliable and readily available of such energy sources. The renewable energy industry has attempted to meet these demands with improved photovoltaic (PV) or solar cells that are less expensive to manufacture, are more reliable in varied use environments, and are more efficient in converting solar energy into electricity. For example, higher efficiency crystalline silicon solar cells are under development, as are lower cost, thin-film solar cells. As materials research and cell design progresses, it is important to be able to accurately test manufactured solar cells to determine whether the materials should be pursued and whether a new design is useful for meeting its intended function.
Regardless of the design and active material used, solar or PV cells are generally subjected to the same standard tests including a test to determine quantum efficiency (QE) of the cell. Quantum efficiency indicates the ratio of the number of charge carriers or electrons generated and collected by the solar cell to the number of photons incident on a solar cell. A QE measurement is critical during cell research and manufacture because cell QE provides an accurate indication of whether a cell is properly designed or operating with an optimal desired spectral response. Such detailed information is obtained with a QE measurement, typically obtained for a full spectrum of light ranging from light in the ultraviolet (UV) range to light in the infrared (IR) range. A graph of the QE measurement at various wavelengths in the light spectrum is useful for indicating if the cell is operating as desired or with a desired spectral response. For example, if the cell's spectral response is suppressed over large portions of the visible light spectrum where the intensity from the sun is high, the tested cell likely cannot convert light to energy with a desired or high efficiency. Based on the QE measurements, solar cell designers can identify material or design defects and can modify the manufacturing process and/or cell design to produce high efficiency solar or PV cells.
The measurement of QE of a solar cell is presently a slow process requiring relatively expensive test stations or equipment. In conventional QE measurements, a meter such as an ammeter is connected to a solar cell to determine generation of charge carriers or electron flow in response to light striking the solar cell. The light or photons are provided in a controlled manner with a light source that includes a white light source, a monochromator, and optical components such as mirrors that direct light from the monochromator onto a small (e.g., 1 to 3 mm2) area of the cell. The monochromator is a relatively expensive device that includes a grating or prism or set of filters for separating the white light into a plurality of wavelengths representing the full light spectrum (e.g., a simulator of standard sunlight of an air mass of 1.5 or AM1.5). A slit at the output of the monochromator is then typically used to restrict the optical path such that only light at or near a particular wavelength is directed toward the mirror and ultimately onto the solar cell. The grating is moved by a motor to disperse in a step-wise manner each of the wavelengths of the light exiting the monochromator slit.
In this manner, the current measurement by the ammeter can be used to determine the QE in a serial manner as each individual wavelength or wavelength range is provided individually by the monochromator. The motor on the monochromator has to provide very accurate movements or control over grating or filter or prism for the light source to provide light one wavelength at a time, and these and other factors result in a typical monochromator used for QE measurement being expensive to purchase and maintain. Additionally, the use of a monochromator results in a relatively slow QE measurement as the monochromator is operated to focus each portion of a full spectrum onto the solar cell followed by determination of the QE at that particular wavelength. A conventional QE measurement may take up to twenty minutes or more to complete. Hence, there is an ongoing demand for less time-consuming techniques for determining the QE of solar or PV cells that are also less expensive to implement.
Additionally, a faster measurement of QE enables a spatial scanning of spectral response properties over a large area of a solar cell. In this way, maps of QE over all or part of a solar cell's area may be produced. Such characterization can identify both systematic and random defects associated with solar cell materials processing, and can be critical in improving the manufacturing yield as part of statistical process controls.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.